The acting head of the Centers for Disease Control and Prevention said yesterday that there is no single best way for communities to respond to the growing outbreak of swine flu, and he played down the role the nation's chief public health agency will play in managing the illness around the country.

"The actions will vary by community, and that's a good thing," said CDC Acting Director Richard Besser. "We'll learn from that what things are more effective and what things are not a good use of resources."

Illustrating the range of responses, 300 schools have canceled classes in an effort to prevent the spread of the virus, though the vast majority are open. Texas Gov. Rick Perry (R) has suspended all high school sports competitions in his state until May 11, while most public officials elsewhere have maintained a wait-and-see attitude.

One consistency, from the Obama administration down, was the effort to convey information without spreading panic.

"I know people are concerned and some people are afraid," Besser said. "And it's important that we do what we can to take those concerns and fears and channel them into personal action and personal planning."

He was certain, however, that the number of cases will rise steeply as the government distributes lab kits that will allow states to test hundreds or thousands of people.

"As we continue to look, we will see more cases in more states," he said. "Unfortunately, I do expect there to be more deaths."

According to the CDC, there were 109 confirmed U.S. cases yesterday in 11 states.

Maryland has nine probable cases, and Virginia reported two confirmed cases last night. The District has reported none.

The number of affected countries increased from nine to 11, with the Netherlands and Switzerland each reporting a case. Worldwide, there were 266 confirmed cases.

However, Keiji Fukuda, the World Health Organization's assistant director general, cautioned against drawing too much from the WHO's raising of the pandemic alert level two notches this week. The move, he said, "is a warning to countries and the global population that the risk of this spreading to their countries is there." It does not urge any particular response on countries.

European Union health ministers, meeting in Luxembourg, rejected a suggestion from France that European countries suspend their airline flights to Mexico. The Czech health minister, Daniela Filipiova, told a news conference that decisions on flights would be left to individual governments in the 27-nation union.

Meanwhile, Vice President Biden told a talk-show host yesterday that he would not want members of his family to get on an airplane now.

Asked whether people should avoid traveling, Besser said no -- with the exception of anybody who is feeling ill.

"I think flying is safe. Going on the subway is safe. People should go out and live their lives," he said.

The federal government's 227-page National Strategy for Pandemic Influenza Implementation Plan, published in May 2006, recommends an "immediate" action at this stage of an outbreak to "limit non-essential passenger travel in affected areas" -- a policy that is not being followed.

As he has in nearly every public appearance, Besser emphasized that some uncertainty and contradictory information are expected. "The microbes don't read the plan, and you need to move away from the plan pretty soon after Day One," he said.

The lack of a strongly prescriptive stance by the government reflects how little is known about what works to slow a pandemic or dampen its effects.

Much of the evidence comes from comparing the experiences of states, cities and villages during the Spanish flu epidemic of 1918-19, which killed at least 50 million people. Many instituted dramatic policies to increase "social distancing" -- essentially, to keep people apart -- and isolate the ill, but the value of such measures wasn't clear.

Arnold Monto, an epidemiologist at the University of Michigan, said public health experts are not sure how aggressive officials should be.

"Are we going to close lots of schools and tell people to work at home? A lot of this is based on a question that we don't have an answer to yet, which is: How severe is this going to be?" Monto said. "We don't want to add social disruption to the already severe problems of having a lot of people sick for a long time."

Thomas Inglesby, deputy director of the Center for Biosecurity at the University of Pittsburgh Medical Center, said week-long closures of individual schools where confirmed cases have occurred seems reasonable, but more wide-scale closures for longer periods may not be effective.

"What's not as obvious is if we close schools, does that reduce the risk of getting flu or not? . . . There's a matter of debate about how strong the evidence is whether school closures really reduce the impact of an epidemic," Inglesby said.

He noted that there are negative repercussions of large-scale school closings. Many children get their only good meal of the day at school, and many children with working parents depend on school as a place to go.

As the epidemic develops, several experts noted that the steps individual communities take will continue to vary.

"The things you do in a rural farm town are going to be very different from what you do in a New York City," said Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "You need to tailor your efforts to the local setting."